Circuits for determining the status of a fan are generally known. However, these circuits have often determined the status of a fan by conditions associated with, or created by, the fan and not by inspection of the fan itself. For instance, in heating systems the status of a fan is often determined by a circuit monitoring the ambient temperature of a chamber. Other systems may have circuits which monitor air flow. Although this may be satisfactory, depending on the use, the indication of a failure by such circuits may not be as a direct result of a fan failure. For instance, an air supply duct may be blocked thereby reducing the amount of air flow. A circuit designed to sense air flow would indicate this as a fault, however the fault would not be a result of a fan failure.
In electronic systems, fan failure is often detected by microswitches, which again sense air flow within the system. It has been found that these microswitches are unreliable as they are susceptible to mechanical malfunctions.
A method of directly monitoring a fan to determine its operating status would, consequently, be more accurate and more reliable.